一个人如何“超越”？ Scala 中的内部类？
在 Enumeration#Val 类的 Scaladoc 中，我可以读到：“实现 Value 类型的类。可以重写该类以更改枚举的命名和整数标识行为。”我很困惑：如何重写一个类？不允许像 override class Val extends super.Val 这样的事情。
In the Scaladoc of class Enumeration#Val, I can read: "A class implementing the Value type. This class can be overridden to change the enumeration's naming and integer identification behaviour." I am puzzled: how do I override a class? Things like override class Val extends super.Val are not permitted.
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Scala 中还没有虚拟类，因此您无法编写
override class Val ...，然后确保调用new Val将动态选择新实例的正确类。相反，会发生的情况是，将根据对封闭类实例的引用类型（在本例中为Enumeration）来选择该类。模拟虚拟类的一般技巧是编写
class Val extends super.Val，然后重写用作类实例工厂的受保护方法。在这种情况下，您还必须重写该方法：Enumeration将仅使用此工厂方法创建Val的实例。一般来说，这种模式需要程序员遵守纪律，但可以通过将构造函数声明为私有来确保，从而强制程序员使用工厂方法。There are no virtual classes in Scala (yet), so you can't write
override class Val ..., and then be sure that invokingnew Valwill dynamically choose the right class for the new instance. What would happen instead is that the class would be chosen based on the type of the reference to the instance of the enclosing class (in this case,Enumeration).The general trick to emulate virtual classes is to write
class Val extends super.Val, and then override a protected method which serves as a factory for instances of the class. In this case, you would also have to override the method:Enumerationwill create instances ofValonly using this factory method. In general, this pattern requires discipline on the programmer's part, but can be ensured by declaring the constructors private, forcing the programmer to use the factory method.